。
She cried out in surprise.
Literal
She [topic-は] startled-and screamed.
びっくり is an onomatopoeic adverb from the family of 擬態語 (state mimetics), capturing the sudden start of being surprised. It pairs with する to form びっくりする ('to be surprised'), one of the everyday verbs of reaction — more vivid and casual than the formal 驚く ('to be astonished'). The te-form here links surprise to action: surprise → cry out, sequenced as cause and reaction.